[DRBD-user] DRBD + LVM

Lars Ellenberg lars.ellenberg at linbit.com
Wed Sep 8 14:46:04 CEST 2010

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

On Thu, Sep 02, 2010 at 09:45:58AM +0930, Mike Hall wrote:
> I'm currently testing a replicated network storage device using CentOS
> 5.5 and DRBD 8.3, as a storage back end for KVM virtualisation. We may
> use either iSCSI (~SAN) or NFS (~NAS) to make this storage available
> to the VM host.

Typical usage scenario.

> I was having trouble changing the status of a DRBD node or bringing a
> node off-line, getting messages about something holding the device
> open. This turned out to be LVM, and it appears that it is necessary
> to bring LVM volumes offline (vgchange -an) before changing DRBD's
> status.

For LVM on top of DRBD (drbdX being a PV), this is correct.

> So, my question is this:
> If I bring LVM volumes offline on node 1 and demote node 1 to
> secondary, then promote node 2 to primary and bring the LVM volumes
> online again on node 2, is it necessary to also bring those LVM
> volumes online again back on node 1?
> I'm guessing no (because it's the 'same' device on either node), but am checking to be sure.

In a "single master" setup (always recommended, unless you have a really
good reason to use dual-master, and know what you are doing),
you cannot bring them online on a DRBD Secondary.
At least you should not be able to. If you can activate your VG despite
it being on top of DRBD, and  DRBD being Secondary, you screwed up your
filters in lvm.conf.

> Also, it it better to:

There is no better.
There is only better for a specific purpose, to a specific measure ;-)

> - format the LVMs on the storage machine (NAS/SAN) and then make those volumes available to the VM host, or
> - in the case of iSCSI, just make unformatted devices available and format them from the VM host?

In this case, I think it does not make much difference.

> Anything else I need to be aware of ?

: Lars Ellenberg
: LINBIT | Your Way to High Availability
: DRBD/HA support and consulting http://www.linbit.com

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